How to Case in an Exterior Door

Measure the width of the door's bottom edge. Transfer this measurement to the piece of casing. Cut the casing straight across at the mark using a miter saw.

Lay the casing face down and run a bead of silicone caulk near the outer edge. Position the casing underneath the door. Secure it with evenly spaced, 2-inch-long galvanized finishing nails from a pneumatic nail gun.

Measure the height from the door's top edge to the bottom casing's bottom edge, on both the right and left side of the door. Transfer these measurements to two pieces of exterior door casing and cut them to fit. Lay both pieces of casing face down. Apply a bead of silicone caulk near the casing's outer edge.

Position one piece of casing next to the door opening. Hold a level against the casing and position it so it's plumb with the door opening. Attach it with galvanized finishing nails. Repeat the process with the second piece of casing on the opposite side of the door.

Measure the distance from the left-side casing's outer edge to the right-side casing's outer edge. Transfer this measurement to a fourth piece of casing. Cut the casing straight across with a miter saw.

Lay the casing face down and run a bead of caulk along its bottom edge. Set the casing in place over the door. Its bottom edge should rest against the top edges of the two side casings as well as the Z-strip installed above the door. Secure it with galvanized finishing nails.

Run a bead of caulk around the door casing's perimeter, where the casing meets the house's siding.